Import volumes in the major US retail container ports is expected to grow 5.1% year-on-year during September as retailers head into the holiday season, new research has found.

The monthly Global Port Tracker report released by the National Retail Federation (NRF) and Hackett Associates said US ports handled 1.43m Twenty-Foot Equivalent Units (TEUs) in July, the latest month for which after-the-fact numbers are available, a 5.4% increase over June and 1.1% up on July.

"Retailers are making up for the slow imports seen earlier in the year," NRF vice president for supply chain and customs policy Jonathan Gold said. "It's too early to predict holiday sales, but merchants are clearly stocking up."

August was estimated at 1.48m TEU, up 4.1% from last year; September is forecast at 1.48m TEU, a 5.1% increase; while the tracker is forecasting a 9% increase in October to reach 1.46m TEU. It expects a 2.2% increase in November at 1.31m TEU, a 0.7% increase in December to 1.3m TEU. January is forecast to reach 1.33m TEU, up 1.9% on the prior year period.

"The US economy is on the road to sustained growth," said Hackett Associates founder Ben Hackett.

"Second-quarter GDP was well above expectations and surprised most forecasters, the unemployment picture is improving, and we believe consumer confidence will translate into increased sales during the fourth quarter."